Key exchange method and key exchange system

ABSTRACT

Plurality of users share a common key while permitting change of members sharing the common key and computational complexity required for key exchange is reduced. Ri and ci are computed based on a twisted pseudo-random function in a first key generation step. sid is generated based on a target-collision resistant hash function and (sid, Rα, Rβ) is transmitted to communication devices Ui in a session ID generation step. T1 and T′ are computed based on a pseudo-random function in a representative second key generation step. Tj is computed based on the pseudo-random function in a general second key generation step. k′ is computed based on the twisted pseudo-random function and T′j is computed with respect to each j in a third key generation step. K1l and k1 are computed in a first session key generation step. A common key K2 is generated based on the pseudo-random function in a second session key generation step.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of and claims the benefit of priorityunder 35 U.S.C. § 120 from application Ser. No. 16/321,657 filed Jan.29, 2019, the entire contents of which are incorporated herein byreference.

U.S. application Ser. No. 16/321,657 is a National State ofPCT/JP2017/031188 filed Aug. 30, 2017, which claims the benefit ofpriority under 35 U.S.C. § 119 from Japanese Application No. 2016-170900filed Sep. 1, 2016.

TECHNICAL FIELD

The present invention relates to an application of an informationsecurity technology, and especially relates to a key exchange technologyby which a plurality of users forming a group share a common key.

BACKGROUND ART

The key exchange technology by which a plurality of users forming agroup share a common key has been conventionally proposed (seeNon-patent Literatures 1 and 2, for example). An architecture of aninformation system for realizing such key exchange technology isdescribed in Non-patent Literature 1. An algorithm for such key exchangetechnology is described in Non-patent Literature 2.

PRIOR ART LITERATURE Non-Patent Literature

-   Non-patent Literature 1: Suvo Mittra, “Iolus: a framework for    scalable secure multicasting”, SIGCOMM '97, pp. 277-288-   Non-patent Literature 2: “Scalable Multicast Key Distribution”,    [online], [retrieved on Jul. 12, 2016], internet    <URL:https://tools.ietf.org/html/rfc1949>

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In the related-art technologies described in Non-patent Literatures 1and 2, it is impossible for a plurality of users to share a common keywhile permitting dynamic member change such as to newly share the commonkey with users other than those already sharing the common key.

Further, since the whole computational complexity required for keyexchange is O(log n) when the number of users is denoted as n, there isa problem that the computational complexity for the key exchange isincreased along with increase of the number of users.

In view of this problem, an object of the present invention is toprovide a key exchange technology which enables a plurality of users toshare a common key while permitting change of members who share thecommon key and enables reduction in computational complexity requiredfor key exchange.

Means to Solve the Problems

According to an aspect of the present invention, n denotes an integerwhich is 2 or larger, i denotes each integer from 1 to n, j denotes eachinteger from 2 to n, S denotes a key distribution device, U_(i) denotesn pieces of communication devices, U₁ denotes one piece ofrepresentative communication device which is selected from thecommunication devices U_(i), U_(j) denotes n−1 pieces of generalcommunication devices obtained by excluding the representativecommunication device U₁ from the communication devices U_(i), ∥ denotesa concatenation operator, α and β are defined by following formulas,

$\alpha = \left\{ {\begin{matrix}{i - 1} & {{{{if}\ i} - 1} \geq 1} \\{i - 1 + n} & {{{{if}\ i} - 1} < 1}\end{matrix},{\beta = \left\{ \begin{matrix}{i + 1} & {{{{if}\ i} + 1} \leq n} \\{i + 1 - n} & {{{{if}\ i} + 1} > n}\end{matrix} \right.}} \right.$

secret strings st_(S) and st′_(S) are stored in a storage of the keydistribution device S, and secret strings st_(i) and st′_(i) are storedin a storage of the communication devices U_(i). The aspect includes: afirst key generation step in which the communication devices U_(i)generate r_(i), k_(i), and s_(i) by using the secret strings st_(i) andst′_(i) based on a twisted pseudo-random function, compute R_(i)=g^(r_i)and c_(i)=g^(k_i)h^(s_i), and transmit (R_(i), c_(i)) to the keydistribution device S; a session ID generation step in which the keydistribution device S generates sid by using c₁, . . . , c_(n) based ona target-collision resistant hash function and transmits, to thecommunication devices U_(i), (sid, R_(α), R_(β)) with respect to each i;a representative second key generation step in which the representativecommunication device U₁ generates K₁ ^(l) by using (sid, R_(n) ^(r_1))based on a pseudo-random function, generates K₁ ^(r) by using (sid, R₂^(r_1)) based on the pseudo-random function, computes T₁ by an exclusiveOR of K₁ ^(l) and K₁ ^(r), computes T′ by an exclusive OR of K₁ ^(l) andk₁∥s₁, and transmits (T₁, T′) to the key distribution device S; ageneral second key generation step in which the general communicationdevices U_(j) generate K_(j) ^(l) by using (sid, R_(α) ^(r_j)) based onthe pseudo-random function, generate K_(j) ^(r) by using (sid, R_(β)^(r_j)) based on the pseudo-random function, compute T_(j) by anexclusive OR of K_(j) ^(l) and K_(j) ^(r), and transmit (k_(j), s_(j),T_(j)) to the key distribution device S; a third key generation step inwhich the key distribution device S generates k_(s) by using the secretstrings st_(S) and st′_(S) based on the twisted pseudo-random function,computes k′ by an exclusive OR of k₂, . . . , k_(n), k_(s), computesT′_(j) by an exclusive OR of T₁, . . . , T_(j−1) with respect to each j,transmits k′ to the representative communication device U₁, andtransmits (k′, T′_(j), T′) to the general communication devices U_(j); afirst session key generation step in which the general communicationdevices U_(j) compute K₁ ^(l) by an exclusive OR of T′_(j) and K_(j)^(l) and compute k₁∥s₁ by an exclusive OR of T′ and K₁ ^(l); and asecond session key generation step in which the communication devicesU_(i) generate a common key K₂ by using sid and an exclusive OR of k′and k₁ based on the pseudo-random function.

Another aspect of the present invention is a key exchange method for acase where communication devices U_(n+1), . . . , U_(n+k) newly join asession established by communication devices U₁, . . . , U_(n), when ndenotes an integer which is 2 or larger, k denotes an integer which is 1or larger, ∥ denotes a concatenation operator, S denotes a keydistribution device, and U_(i) (i=1, . . . , n+k) denotes n+k pieces ofcommunication devices, in which U₁ denotes one piece of representativecommunication device which is selected from the communication devicesU₁, U_(n), U_(n+1), . . . , U_(n+k), secret strings st_(S) and st′_(S)are stored in a storage of the key distribution device S, secret stringsst_(i) and st′_(S) are stored in a storage of the communication devicesU_(i) (i=1, . . . , n+k), and secret information r, which is generatedin the session established by communication devices U₁, . . . , U_(n),is stored in the storage of the communication devices U₁, . . . , U_(n).The key exchange method includes: a first key generation step in whichthe communication devices U_(i)(i=1, n, . . . , n+k) generate r_(i),k_(i), and s_(i) by using the secret strings st_(i) and st′_(i) based ona twisted pseudo-random function, compute R_(i)=g^(r_i) andc_(i)=g^(k_i)h^(s_i), and transmit (R_(i), c_(i)) to the keydistribution device S, and the communication devices U_(i) (i=2, . . . ,n−1) generate k_(i) and s_(i) by using the secret strings st_(i) andst′_(i) based on the twisted pseudo-random function, computec_(i)=g^(k_i)h^(s_i), and transmit c_(i) to the key distribution deviceS; a session ID generation step in which the key distribution device Sgenerates sid by using c₁, . . . , c_(n+k) based on a target-collisionresistant hash function and transmits, to the communication devicesU_(i), (sid, R_(i−1)) with respect to i=1, 2, sid with respect to i=3, .. . , n−2, (sid, R_(i+1)) with respect to i=n−1, n, and (sid, R_(i−1),R_(i+1)) with respect to i=n+1, . . . , n+k (here, R₀=R_(n+K) andR_(n+k+1)=R₁); a second key generation step in which the representativecommunication device U₁ generates K₁ ^(l) by using (sid, R_(n+k) ^(r_1))based on a pseudo-random function, generates K₁ ^(r) by using (sid,g^(r_1r)) based on the pseudo-random function, computes T₁ by anexclusive OR of K₁ ^(l) and K₁ ^(r), computes T′ by an exclusive OR ofK₁ ^(l) and k₁∥s₁, and transmits (T₁, T′) to the key distribution deviceS, the communication device U₂ generates K₂ ^(l) by using (sid, R₁ ^(r))based on the pseudo-random function, generates K₂ ^(r) by using (sid,g^(r)) based on the pseudo-random function, computes T₂ by an exclusiveOR of K₂ ^(l) and K₂ ^(r), and transmits (k₂, s₂, T₂) to the keydistribution device S, the communication devices U_(i) (i=3, . . . ,n−2) transmit (k_(i), s_(i)) to the key distribution device S, thecommunication device U_(n−1) generates K_(n−1) ^(l) by using (sid,g^(r)) based on the pseudo-random function, generates K_(n−1) ^(r) byusing (sid, R_(n) ^(r)) based on the pseudo-random function, computesT_(n−1) by an exclusive OR of K_(n−1) ^(l) and K_(n−1) ^(r), andtransmits (k_(n−1), s_(n−1), T_(n−1)) to the key distribution device S,the communication device U_(n) generates K_(n) ^(l) by using (sid, R_(n)^(r)) based on the pseudo-random function, generates K_(n) ^(r) by using(sid, R_(n+1) ^(r_n)) based on the pseudo-random function, computesT_(n) by an exclusive OR of K_(n) ¹ and K_(n) ^(r), and transmits(k_(n), s_(n), T_(n)) to the key distribution device S, and thecommunication devices U_(i) (i=n+1, . . . , n+k) generate K_(i) ¹ byusing (sid, R_(i−1) ^(r_i)) based on the pseudo-random function,generate K_(i) ^(r) by using (sid, R_(i+1) ^(r_i)) based on thepseudo-random function, compute T_(i) by an exclusive OR of K₁ ^(l) andK₁ ^(r), and transmit (k_(i), s_(i), T_(i)) to the key distributiondevice S; a third key generation step in which the key distributiondevice S generates k_(s) by using the secret strings st_(S) and st′_(S)based on the twisted pseudo-random function, computes k′ by an exclusiveOR of k₂, . . . , k_(n+k), k_(s), computes T′_(i) by an exclusive OR ofT₁, . . . , T_(i−1) with respect to i=2, . . . , n+k (here, T_(i) is nilwith respect to i=3, . . . , n−1), transmits k′ to the representativecommunication device U₁, and transmits (k′, T′_(i), T′) to thecommunication devices U_(i) (i=2, . . . , n+k); a first session keygeneration step in which the communication devices U_(i) (i=2, n, . . ., n+k) compute K₁ ^(l) by an exclusive OR of T′_(i) and K_(i) ^(l) andcompute k₁∥s₁ by an exclusive OR of T′ and K₁ ^(l), and thecommunication devices U_(i) (i=3, . . . , n−1) compute K₁ ^(l) by anexclusive OR of T′_(i) and g^(r) and compute k₁∥s₁ by the exclusive ORof T′ and K₁ ^(l); and a second session key generation step in which thecommunication devices U_(i) (i=1, . . . , n+k) generate a common key K₂by using sid and an exclusive OR of k′ and k₁ based on the pseudo-randomfunction.

Effects of the Invention

According to the present invention, a plurality of users can share acommon key while permitting change of members who share the common key.Computational complexity required for key exchange is the constantnumber of times which is the number of users, that is, O(1), thus beingreduced compared to the related art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the functional configuration of a key exchangesystem.

FIG. 2A illustrates the functional configuration of a key distributiondevice.

FIG. 2B illustrates the functional configuration of a communicationdevice.

FIG. 3 illustrates a processing flow (system setup) in a key exchangemethod.

FIG. 4 illustrates a processing flow (session key distribution) in thekey exchange method.

FIG. 5 illustrates a processing flow (user addition) in the key exchangemethod.

DETAILED DESCRIPTION OF THE EMBODIMENT

Prior to the description of an embodiment, the notation or the like inthis specification will be described.

Notation

An underscore (_) denotes a subscript. For example, x^(y_z) representsthat y_(z) is a superscript with respect to x and x_(y_z) representsthat y_(z) is a subscript with respect to x.

To select an element n at random from Set which is a certain set isexpressed as m∈_(R)Set.

To output y based on ALG, which is a certain algorithm, with respect toan input x and a random number r is expressed as y←ALG(x;r).

Here, in the case where ALG is a deterministic algorithm, the randomnumber r is nil.

|⋅| denotes the bit length of a value.

Definition

κ denotes a security parameter.

F={F_(κ): Dom_(κ)×FS_(κ)→Rng_(κ)}_(κ) is a family of functions includinga definition range {Dom_(κ)}_(κ), a key space {FS_(κ)}_(κ), and a valuerange {Rng_(κ)}_(κ). In this case, if a function F_(κ) and a true randomfunction RF_(κ): Dom_(κ)→Rng_(κ) cannot be distinguished with respect toa person D to be identified at arbitrary polynomial time, F={F_(κ)}κ iscalled a family of pseudo-random functions. A specific example of thepseudo-random function is described in Reference Literature 1 below, forexample.

-   Reference Literature 1: O. Goldreich, “Modern Cryptography,    Probabilistic Proofs and Pseudo-randomness”, Springer-Verlag Tokyo,    2001

H={H_(κ): Dom_(κ)→Rng_(κ)}_(κ) is a family of hash functions includingthe definition range {Dom_(κ)}_(κ) and the value range {Rng_(κ)}_(κ). Inthis case, if x′ (≠x) which satisfies H_(κ)(x)=H_(κ)(x′) whenx∈_(R)Dom_(κ) is provided cannot be found with respect to an attacker Aat arbitrary polynomial time, H={H_(κ)}_(κ) is called a family oftarget-collision resistant hash functions. A specific example of thetarget-collision resistant hash function is described in ReferenceLiterature 2 below, for example.

-   Reference Literature 2: J. A. Buchmann, “Introduction to    Cryptography—Edition 3”, Maruzen Publishing Co., Ltd., 2007

Message authentication code algorithms are defined as (MGen, Tag, Ver).In a MAC key generation algorithm MGen, the security parameter κ is aninput and a MAC key ink is an output. In the tag generation algorithmTag, the MAC key mk and the plaintext m are inputs and theauthentication tag σ is an output. In a verification algorithm Ver, theMAC key ink, the plaintext in, and the authentication tag σ are inputs,and 1 is outputted when the authentication tag σ is correct, while 0 isoutputted when the authentication tag σ is incorrect. A specific exampleof the message authentication code algorithm is described in ReferenceLiterature 2 mentioned above, for example.

Functional encryption algorithms are defined as (Setup, Der, FEnc,FDec). In a setup algorithm Setup, the security parameter κ is an inputand a master secret key msk and a public parameter Params are outputs.In a key derivation algorithm Der, the public parameter Params, themaster secret key msk, and attribution A are inputs and a user secretkey usk is an output. In an encryption algorithm FEnc, the publicparameter Params, an access structure P, and the plaintext m are inputsand the cipher text CT is an output. In a decryption algorithm FDec, theuser secret key usk and the cipher text CT are inputs and the plaintextin is outputted if the attribution A satisfies the access structure P. Aspecific example of the functional encryption algorithm is described inReference Literature 3 below, for example.

-   Reference Literature 3: D. Boneh, A. Sahai, and B. Waters,    “Functional encryption: definitions and challenges”, TCC, Lecture    Notes in Computer Science, vol. 6597, pp. 253-273, 2011.

ID-base encryption algorithms are defined as (IBE-Setup, IBE-Der,IBE-Enc, IBE-Dec). In a setup algorithm IBE-Setup, the securityparameter κ is an input and a master secret key msk and a publicparameter Params are outputs. In a key derivation algorithm IBE-Der, thepublic parameter Params, the master secret key msk, and ID∈{0, 1}* areinputs and a user secret key usk is an output. In an encryptionalgorithm IBE-Enc, the public parameter Params, ID∈{0, 1}, and theplaintext m are inputs and the cipher text CT is an output. In adecryption algorithm IBE-Dec, the public parameter Params, the usersecret key usk, and the cipher text CT are inputs and the plaintext inis an output. A specific example of the ID-base encryption algorithm isdescribed in Chapter 8 “Boneh-Franklin IBE” and Chapter 9 “Boneh-BoyenIBE” in Reference Literature 4 below, for example.

-   Reference Literature 4: Luther Martin, “Introduction to    Identity-Based Encryption”, 1st Edition, Artech House, January 2008.

A function tPRF: {0, 1}^(κ)×FS_(κ)×FS_(κ)×{0, 1}^(κ)→Rng_(κ) is called atwisted pseudo-random function, andtPRF(a,a′,b,b′):=F _(κ)(a,b)⊕F _(κ)(b′,a′)

is defined by using the pseudo-random function F_(κ). Here, a, b′∈{0,1}^(κ) and a′, b∈FS_(κ). A specific example of the twisted pseudo-randomfunction is described in Reference Literature 5 below, for example.

-   Reference Literature 5: Kazuki Yoneyama, “One-Round Authenticated    Key Exchange with Strong Forward Secrecy in the Standard Model    against Constrained Adversary”, IEICE Transactions, vol. E96-A, no.    6, pp. 1124-1138, 2013.

An embodiment of the present invention will be detailed below.Components having identical functions in the drawings will be denoted byidentical reference characters and duplicate description thereof will beomitted.

System Structure

As illustrated in FIG. 1 , the key exchange system according to theembodiment includes a key distribution device 1 and N (≥2) pieces ofcommunication devices 2 ₁ . . . , 2 _(N). In this embodiment, the keydistribution device 1 and the communication devices 2 ₁, . . . , 2 _(N)are respectively connected to a communication network 3. Thecommunication network 3 is a communication network adopting the circuitswitching system or the packet switching system which is configured sothat the key distribution device 1 can communicate with each of thecommunication devices 2 ₁, . . . , 2 _(N). In this embodiment, thecommunication devices 2 ₁, . . . , 2 _(N) do not have to be able tocommunicate with each other. The communication network 3 does not haveto be a communication path in which safety is ensured but an internet orthe like, for example, can be employed as the communication network 3.

The key distribution device 1 includes a storage 100, a first setup unit101, a second setup unit 102, a user ID reception unit 103, an initialkey generation unit 104, a secret string generation unit 105, a user keytransmission unit 111, a session ID generation unit 113, anauthentication tag verification unit 114, a third key generation unit115, and an authentication tag generation unit 116, as illustrated inFIG. 2A. The communication device 2 includes a storage 200, a user IDtransmission unit 203, an initial key reception unit 204, a secretstring generation unit 205, a user key reception unit 211, a first keygeneration unit 212, a second key generation unit 214, an authenticationtag generation unit 215, an authentication tag verification unit 216,and a session key generation unit 217, as illustrated in FIG. 2B. Thekey distribution device 1 and the communication devices 2 ₁, . . . , 2_(N) perform processing of each step illustrated in FIGS. 3 to 5 ,realizing the key exchange method according to the embodiment.

The key distribution device 1 and the communication devices 2 ₁, . . . ,2 _(N) are special devices which are configured such that a specialprogram is read into well-known or dedicated computers including acentral processing unit (CPU), a main storage device (random accessmemory: RAM), and the like. Each device executes processing under thecontrol of the central processing unit, for example. Data inputted intoeach device and data obtained through each processing are stored in themain storage device, for example, and the data stored in the mainstorage device is read onto the central processing unit as appropriateto be used for other processing. At least part of processing unitsincluded in each device may be composed of hardware such as anintegrated circuit.

The storage 100 included in the key distribution device 1 and thestorage 200 included in the communication devices 2 ₁, . . . , 2 _(N)may be composed of a main storage device such as a random access memory(RAM), an auxiliary storage device composed of a hard disk, an opticaldisk, or a semiconductor memory element such as a flash memory, ormiddleware such as a relational database and a key value store. Sinceeach storage stores secret information, it is preferable that eachstorage is a storage device having tamper resistance (a SIM card, forexample).

System Setup

A processing procedure for system setup in the key exchange methodaccording to the embodiment will be described with reference to FIG. 3 .

In the following description, symbols will be defined as the following.S denotes the key distribution device 1 and U_(i) (i∈{1, . . . , N})denote N pieces of communication devices 2 ₁, . . . , 2 _(N). G denotesa multiplicative cyclic group of a prime number order p of K bits. Eachof g and h denotes a generator of the group G. H: {0, 1}*→{0, 1}^(κ)denotes a target-collision resistant hash function. tPRF: {0,1}^(κ)×FS_(κ)×FS_(κ)×{0, 1}^(κ)→Z_(p) and tPRF′: {0,1}^(κ)×FS_(κ)×FS_(κ)×{0, 1}^(κ)→FS_(κ) denote twisted pseudo-randomfunctions. F: {0, 1}^(κ)×G→Z_(p) ², F′: {0, 1}^(κ)×Z_(p)→FS_(κ), F″: {0,1}^(κ)×FS_(κ)→{0, 1}^(κ), and F′″: {0, 1}^(κ)×FS_(κ)→Z_(p) denotepseudo-random functions.

Here, when S and U_(i) are inputs of each algorithm, S and U_(i) denoteidentifiers for uniquely specifying respective devices.

In step S101, the first setup unit 101 of the key distribution device Sgenerates the public parameter Params and the master secret key mskbased on the setup algorithm Setup for functional encryption. The firstsetup unit 101 transmits the public parameter Params to each of thecommunication devices U₁, . . . , U_(N). The public parameter Params andthe master secret key msk are stored in the storage 100. Further, thecommunication devices U_(i) store the public parameter Params receivedfrom the key distribution device S in the storage 200.

In step S102, the second setup unit 102 of the key distribution device Sgenerates the public parameter Params' and the master secret key msk′ ofthe key distribution device S based on the setup algorithm IBE-Setup forID-base encryption. The public parameter Params' and the master secretkey msk′ are stored in the storage 100.

In step S203, the user ID transmission unit 203 of the communicationdevices U_(i) transmits U_(i) to the key distribution device S so as toregister a user ID in the key distribution device S. Since the user IDmay be an arbitrary letter string which enables identification of auser, a mail address, for example, may be used.

In step S103, the user ID reception unit 103 of the key distributiondevice S receives U_(i) which is a user ID. In step S104, the initialkey generation unit 104 of the key distribution device S generates aninitial key sk_(U_i)←IBE-Der(Params′, msk′, U_(i)) of the communicationdevices U_(i) based on the key derivation algorithm IBE-Der for TD-baseencryption and transmits the initial key sk_(U_i) to the communicationdevices U_(i).

In step S204, the initial key reception unit 204 of the communicationdevices U_(i) receives the initial key sk_(U_i) of the communicationdevices U_(i). The communication devices U_(i) store the initial keysk_(U_i) in the storage 200.

In step S105, the secret string generation unit 105 of the keydistribution device S generates secret strings (st_(S), st′_(S)) used inthe twisted pseudo-random function as st_(S)∈_(R)FS_(κ) and st′_(S)∈{0,1}_(κ). The secret strings (st_(S), st′_(S)) are stored in the storage100.

In step S205, the secret string generation unit 205 of the communicationdevices U_(i) generates secret strings (st_(i), st′_(i)) used in thetwisted pseudo-random function as st_(i)∈_(R)FS_(κ) and st′_(i)∈{0,1}_(κ). The secret strings (st_(i), st′_(i)) are stored in the storage200.

Session Key Distribution

A processing procedure for session key distribution in the key exchangemethod according to the embodiment will be described with reference toFIG. 4 .

In the following description, it is assumed that arbitrary n (≤N) piecesof communication devices U_(i) (i∈{1, . . . , n}) among N pieces ofcommunication devices 2 ₁, . . . , 2 _(N) share a session key SK whichis a common key.

In step S111, in the case where a session is started by thecommunication devices U_(i) and the session is the first session in atime frame TF of the communication devices U_(i), the user keytransmission unit 111 of the key distribution device S generates a usersecret key usk_(i)←Der(Params, msk, A_(i)) of the communication devicesU_(i) based on the key derivation algorithm Der for functionalencryption with current time and attribution respectively used as timeand A_(i)=(U_(i), time). Further, the user key transmission unit 111generates a MAC key mk_(i)←MGen of the communication devices U_(i) basedon the key generation algorithm MGen for a message authentication code.Then, the user key transmission unit 111 encrypts the user secret keyusk_(i) and the MAC key mk_(i) based on the encryption algorithm IBE-Encfor ID-base encryption so as to generate the cipher textCT_(i)←IBE-Enc(Params′, U_(i), (usk_(i), mk_(i))). The user keytransmission unit 111 transmits the cipher text CT_(i) to each of thecommunication devices U_(i).

In step S211, the user key reception unit 211 of the communicationdevices U_(i) decrypts the cipher text CT_(i), which is received fromthe key distribution device S, by using the initial key sk_(U_i) of thecommunication devices U_(i) based on the decryption algorithm IBE-Decfor ID-base encryption so as to obtain a user secret key and a MAC key(usk_(i), mk_(i))←IBE-Dec_(sk_Ui)(CT_(i), Params′). The user keyreception unit 211 stores the user secret key usk_(i) and the MAC keymk_(i) in the storage 200.

In step S212, the first key generation unit 212 of the communicationdevices U_(i) generates ^(˜)r_(i)∈_(R){0, 1}^(κ), ^(˜)r′_(i)∈_(R)FS_(κ),^(˜)k_(i)∈_(R){0, 1}^(κ), ^(˜)k′_(i)∈_(R)FS_(κ), ^(˜)s_(i)∈_(R){0,1}^(κ), and ^(˜)s′_(i)∈_(R)FS_(κ) and computes r_(i)=tPRF(^(˜)r_(i),^(˜)r′_(i), st_(i), st′_(i)), k_(i)=tPRF(^(˜)k_(i), ^(˜)k′_(i), st_(i),st′_(i)), and s_(i)=tPRF(^(˜)s_(i), ^(˜)s′_(i), st_(i), st′_(i)) basedon the twisted pseudo-random function tPRF. Further, the first keygeneration unit 212 computes R_(i)=g^(r_i) and c_(i)=g^(k_i)h^(s_i).Then, the first key generation unit 212 transmits (R_(i), c_(i)) to thekey distribution device S.

In step S112, the key distribution device S receives (R_(i), c_(i)) fromthe communication devices U_(i). At this time, the key distributiondevice S stands by until the key distribution device S receives (R₁,c₁), . . . , (R_(n), c_(n)) respectively from all of the communicationdevices U₁, . . . , U_(n).

In step S113, the session ID generation unit 113 of the key distributiondevice S generates sid=H(c₁, . . . , c₁) by using c₁, . . . , c_(n),which are respectively received from the communication devices U₁, . . ., U_(n), based on the target-collision resistant hash function H.Further, the session ID generation unit 113 selects one piece ofcommunication device as a representative from n pieces of communicationdevices U₁, . . . , U_(n). A representative may be arbitrarily selected.For example, a predetermined communication device with the highestpriority or a communication device which has started a session mostrecently may be selected. It is assumed that the communication device U₁is selected, and U₁ is called a representative communication device inthis example. Further, n−1 pieces of communication devices U_(j) (j∈{2,. . . , n}) other than the representative communication device U₁ arecalled general communication devices. The session ID generation unit 113computes α and β as the following formulas and transmits (sid, R_(α),R_(β)) to each of the communication devices U_(i). Further, the keydistribution device S notifies U₁ that U₁ is the representativecommunication device.

$\alpha = \left\{ {\begin{matrix}{i - 1} & {{{{if}\ i} - 1} \geq 1} \\{i - 1 + n} & {{{{if}\ i} - 1} < 1}\end{matrix},{\beta = \left\{ \begin{matrix}{i + 1} & {{{{if}\ i} + 1} \leq n} \\{i + 1 - n} & {{{{if}\ i} + 1} > n}\end{matrix} \right.}} \right.$

In step S213, each of the communication devices U_(i) receives (sid,R_(α), R_(β)) from the key distribution device S. The communicationdevices U_(i) execute the following processing as soon as thecommunication devices U_(i) receive (sid, R_(α), R_(β)). In the casewhere i=2, . . . , n holds, that is, the case where the communicationdevices U_(i) are the communication devices U_(j) (i=j), the processingis progressed to step S214 _(j). In the case where i=1 holds, that is,the case where the communication device U_(i) is the representativecommunication device U₁, the processing is progressed to step S214 ₁.

In step S214 _(j), the second key generation unit 214 of the generalcommunication devices U_(j) generates K_(j) ^(l) by using (sid, R_(α)^(r_i)) based on the pseudo-random function F and generates K_(j) ^(r)by using (sid, R_(β) ^(r_i)) based on the pseudo-random function F so asto compute T_(j) by an exclusive OR of K_(j) ^(l) and K_(j) ^(r), as thefollowing formulas.K _(j) ^(l) =F(sid,R _(α) ^(r) ^(j) ),K _(j) ^(r) =F(sid,R _(β) ^(r) ^(j) ),T _(j) =K _(j) ^(l) ⊕K _(j) ^(r)

In step S215 _(j), the authentication tag generation unit 215 of thegeneral communication devices U_(j) generates an authentication tagσ_(j)=Tag_(mk_j)(R_(j), c_(j), R_(α), R_(β), k_(j), s_(j), T_(j), U_(j),sid) by using the MAC key mk_(j) based on the tag generation algorithmTag for a message authentication code. The authentication tag generationunit 215 transmits (k_(j), s_(j), T_(j), σ_(j)) to the key distributiondevice S.

In step S214 ₁, the second key generation unit 214 of the representativecommunication device U₁ generates K₁ ^(l) by using (sid, R_(n) ^(r_1))based on the pseudo-random function F and generates K₁ ^(r) by using(sid, R₂ ^(r_1)) based on the pseudo-random function F so as to computeT₁ by the exclusive OR of K₁ ^(l) and K₁ ^(r) and compute T′ by theexclusive OR of K₁ ^(l) and k₁∥s₁, as the following formulas. Here, ∥denotes a concatenation operator.K ₁ ^(l) =F(sid,R _(n) ^(r) ¹ ),K ₁ ^(r) =F(sid,R ₂ ^(r) ¹ ),T ₁ =K ₁ ^(l) ⊕K ₁ ^(r)T′=K ₁ ^(l)⊕(k ₁ ∥s ₁)

In step S215 ₁, the authentication tag generation unit 215 of therepresentative communication device U₁ generates an authentication tagσ₁=Tag_(mk_1)(R₁, c₁, R_(n), R₂, T₁, T′, U₁, sid) by using the MAC keymk₁ based on the tag generation algorithm Tag for a messageauthentication code. The authentication tag generation unit 215transmits (T₁, T′, σ₁) to the key distribution device S.

In step S114 _(j), the authentication tag verification unit 114 of thekey distribution device S receives (k_(j), s_(j), T_(j), σ_(j)) from thegeneral communication devices U_(j) with respect to j=2, . . . , n andverifies Ver_(mk_j)(R_(j), c_(j), R_(α), R_(β), k_(j), s_(j), T_(j),U_(j), sid, σ_(j)) by using the MAC key mk_(j) of the generalcommunication devices U_(j) based on the verification algorithm Ver fora message authentication code. When the authentication tag σ_(j) isinvalid, the authentication tag verification unit 114 ends the sessionof the general communication devices U_(j). Further, the authenticationtag verification unit 114 verifies whether or not c_(j)=g^(k_j)h^(s_j)is satisfied with respect to j=2, . . . , n. When c_(j)=g^(k_j)h^(s_j)is not satisfied, the authentication tag verification unit 114 ends thesession of the general communication devices U_(j).

In step S114 ₁, the authentication tag verification unit 114 of the keydistribution device S receives (T₁, T′, σ₁) from the representativecommunication device U₁ and verifies Ver_(mk_1)(R₁, c₁, R_(n), R₂, T₁,T′, U₁, sid, σ₁) by using the MAC key mk₁ of the representativecommunication device U₁ based on the verification algorithm Ver for amessage authentication code. When the authentication tag σ₁ is invalid,the authentication tag verification unit 114 ends the session of therepresentative communication device U₁.

In step S115 a, the third key generation unit 115 of the keydistribution device S generates ^(˜)k_(S)∈_(R){0, 1}^(κ),^(˜)k′_(S)∈_(R)FS_(κ), ^(˜)K₁∈_(R){0, 1}^(κ), and ^(˜)K′₁∈_(R)FS_(κ) soas to compute k_(S)=tPRF(^(˜)k_(S), ^(˜)k′_(S), st_(S), st′_(S)) andK₁=tPRF′(^(˜)K₁, ^(˜)K′₁, st_(S), st′_(S)) based on the twistedpseudo-random function tPRF. Further, the third key generation unit 115computes k′ by the following formula.k′=(⊕_(2≤j≤n) k _(j))⊕k _(S)

In step S115 b, the third key generation unit 115 of the keydistribution device S computes T′_(j) with respect to j=2, . . . , n bythe following formula.T′ _(j)⊕_(1≤i≤j−1) T _(i)

In step S115 c, the third key generation unit 115 of the keydistribution device S encrypts a common key K₁ with respect to i=1, . .. , n based on the encryption algorithm FEnc for functional encryptionwith the access structure P_(i)=(ID=U_(i)){circumflex over ( )}(time∈TF)so as to generate a cipher text CT′_(i)=FEnc(Params, P_(i), K₁). Here,ID is a predicate variable representing a communication device and TF isa predicate variable representing a time frame of the communicationdevice.

In step S116 _(j), the authentication tag generation unit 116 of the keydistribution device S generates an authentication tagσ′_(j)=Tag_(mk_j)(R_(j), c_(j), R_(α), R_(β), k_(j), s_(j), T_(j),U_(j), sid, c₁, k′, T′_(j), T′, CT′_(j)) with respect to j=2, . . . , nby using the MAC key mk_(j) of the general communication devices U_(j)based on the tag generation algorithm Tag for a message authenticationcode. The authentication tag generation unit 116 transmits (c₁, k′,T′_(j), T′, CT′_(j), σ′_(j)) to the general communication devices U_(j).

In step S116 ₁, the authentication tag generation unit 116 of the keydistribution device S generates an authentication tag σ′₁=Tag_(mk_1)(R₁,c₁, R_(n), R₂, T₁, T′, U₁, sid, k′, CT′₁) by using the MAC key ink, ofthe representative communication device U₁ based on the tag generationalgorithm Tag for a message authentication code. The authentication taggeneration unit 116 transmits (k′, CT′₁, σ′₁) to the representativecommunication device U₁.

In step S216 _(j), the authentication tag verification unit 216 of thegeneral communication devices U_(j) receives (c₁, k′, T′_(j), T′,CT′_(j), σ′_(j)) from the key distribution device S and verifiesVer_(mk_j)(R_(j), c_(j), R_(α), R_(β), k_(j), s_(j), T_(j), U_(j), sid,c₁, k′, T′_(j), T′, CT′_(j), σ′_(j)) by using the MAC key mk_(j) of thegeneral communication devices U_(j) based on the verification algorithmVer for a message authentication code. When the authentication tagσ′_(j) is invalid, the authentication tag verification unit 216 ends thesession of the general communication devices U_(j). Further, theauthentication tag verification unit 216 computes K₁ ^(l) by anexclusive OR of T′_(j) and K_(j) ^(l) and computes k₁∥s₁ by an exclusiveOR of T′ and K₁ ^(l), as the following formulas.K ₁ ^(l) =T′ _(j) ⊕K _(j) ^(l),k ₁ ∥s ₁ =T′⊕K ₁ ^(l)

Then, the authentication tag verification unit 216 verifies whether ornot c₁=g^(k_1)h^(s_1) is satisfied. When c₁=g^(k_1)h^(s_1) is notsatisfied, the authentication tag verification unit 216 ends the sessionof the general communication devices U_(j).

In step S216 ₁, the authentication tag verification unit 216 of therepresentative communication device U₁ receives (k′, CT′₁, σ′₁) from thekey distribution device S and verifies Ver_(mk_1)(R₁, c₁, R_(n), R₂, T₁,T′, U₁, sid, k′, CT′₁, σ′₁) by using the MAC key mk₁ of therepresentative communication device U₁ based on the verificationalgorithm Ver for a message authentication code. When the authenticationtag σ′₁ is invalid, the authentication tag verification unit 216 endsthe session of the representative communication device U₁.

In step S217, the session key generation unit 217 of the communicationdevices U_(i) decrypts the common key K₁←FDec_(usk_i)(CT′_(i), P_(i)) byusing the user secret key usk_(i) of the communication devices U_(i)based on the decryption algorithm FDec for functional encryption.Further, the session key generation unit 217 computes a common key K₂based on the pseudo-random function F′ as the following formula.K ₂ =F′(sid,k′⊕k ₁)

Then, the session key generation unit 217 computes a session key SKbased on the pseudo-random function F″ as the following formula.SK=F″(sid,K ₁)⊕F″(sid,K ₂)

According to the key exchange technology of the present invention, aplurality of users can share a common key by preliminarily registeringonly user IDs as information of users who may perform key exchange, inthe above-described configuration. Especially, a plurality of users canshare the common key K₂ and the session key SK without leaking anyinformation other than user IDs with respect to the key distributiondevice S. Further, the whole computational complexity required for thekey exchange has been O(log n) when the number of users is set as n inthe related art, but according the present invention, the wholecomputational complexity is the constant number of times which is thenumber of users, that is, O(1), enabling key exchange with smallercomputational complexity than the related art.

Further, since a key can be issued by using existing IDs based onID-base encryption, a public key database which is required for generalpublic key encryption does not need to be prepared, reducing operationalcost advantageously.

A processing procedure of the key exchange method in new user additionto a plurality of users sharing a session key will be described below.It is assumed that the session key SK has been already shared among thecommunication devices U_(j) (i∈{1, . . . , n}). Each of thecommunication devices U_(i) stores secret information to be used foruser addition in the storage 200 after the end of the key exchangeprocessing (after S217). The representative communication device U₁stores secret information H₁ ^(l), H₁ ^(r), and r computed by thefollowing formulas in the storage 200.H ₁ ^(l) =R _(n) ^(r) ¹H ₁ ^(r) =R ₂ ^(r) ¹r=F′″(sid,K ₁)⊕F′″(sid,K ₂)

The general communication devices U_(j) (j∈{2, . . . , n}) store secretinformation H_(j) ¹, H_(j) ^(r), and r computed by the followingformulas in the storage 200.H _(j) ^(l) =R _(i−1) ^(r) ^(j)H _(j) ^(r) =R _(i+1) ^(r) ^(j)r=F′″(sid,K ₁)⊕F′″(sid,K ₂)

User Addition

A processing procedure of user addition in the key exchange methodaccording to the embodiment will be described with reference to FIG. 5 .

It is assumed that communication devices U_(n+1), . . . , U_(n+k) (k isan integer which is 1 or larger and n+k is an integer which is N orsmaller) newly join a session established by the communication devicesU₁, . . . , U_(n). Hereinafter, it is assumed that i∈{1, . . . , n+k}holds.

In step S311, in the case where a session is started by thecommunication device U_(i) (i=1, . . . , n+k) and the session is thefirst session in a time frame TF′ of the communication devices U_(i),the user key transmission unit 111 of the key distribution device Sgenerates a user secret key usk_(i)←Der(Params, msk, A_(i)) of thecommunication device U_(i) based on the key derivation algorithm Der forfunctional encryption, with current time and attribution respectivelyused as time and A_(i)=(U_(i), time). Further, the user key transmissionunit 111 generates a MAC key mk_(i)←MGen of the communication deviceU_(i) based on the key generation algorithm MGen for a messageauthentication code. Then, the user key transmission unit 111 encryptsthe user secret key usk_(i) and the MAC key mk_(i) based on theencryption algorithm IBE-Enc for ID-base encryption so as to generatethe cipher text CT_(i)←IBE-Enc(Params′, U_(i), (usk_(i), mk_(i))). Theuser key transmission unit 111 transmits the cipher text CT_(i) to eachof the communication devices U_(i).

In step S411, the user key reception unit 211 of the communicationdevices U_(i) decrypts the cipher text CT_(i), which is received fromthe key distribution device S, by using the initial key sk_(U_i) of thecommunication devices U_(i) based on the decryption algorithm IBE-Decfor ID-base encryption so as to obtain a user secret key and a MAC key(usk_(i), mk_(i))←IBE-Dec_(sk_Ui)(CT_(i), Params′). The user keyreception unit 211 stores the user secret key usk_(i) and the MAC keymk_(i) in the storage 200.

In step S412, in the case of i∈{1}∪[n, n+k], the first key generationunit 212 of the communication device U_(i) generates ^(˜)r_(i)∈_(R){0,1}^(κ), ^(˜)r′_(i)∈_(R)FS_(κ), ^(˜)k_(i)∈R{0, 1}^(κ),^(˜)k′_(i)∈_(R)FS_(κ), ^(˜)s_(i)∈_(R){0, 1}^(κ), and^(˜)s′_(i)∈_(R)FS_(κ) so as to compute r_(i)=tPRF(^(˜)r_(i), ^(˜)r′₁,st_(i), st′_(S)), k_(i)=tPRF(^(˜)k_(i), ^(˜)k′_(i), st_(i), st′_(i)),and s_(i)=tPRF(^(˜)s_(i), ^(˜)s′_(i), st_(i), st′_(i)) based on thetwisted pseudo-random function tPRF. Further, the first key generationunit 212 computes R_(i)=g^(r_i) and c_(i)=g^(k_i)h^(s_i). Then, thefirst key generation unit 212 transmits (R_(i), c_(i)) to the keydistribution device S.

In the case of i∈[2, n−1], the first key generation unit 212 of thecommunication devices U_(i) generates ^(˜)k_(i)∈_(R){0, 1}^(κ),^(˜)k′_(i)∈_(R)FS_(κ), ^(˜)s_(i)∈_(R){0, 1}^(κ), and^(˜)s′_(i)∈_(R)FS_(κ) and computes k_(i)=tPRF(^(˜)k_(i), ^(˜)k′_(i),st_(i), st′_(i)) and s_(i)=tPRF(^(˜)s_(i), ^(˜)s′_(i), st_(i), st′_(i))based on the twisted pseudo-random function tPRF. Further, the first keygeneration unit 212 computes c_(i)=g^(k_i)h^(s_i). Then, the first keygeneration unit 212 transmits c_(i) to the key distribution device S.

In step S312, the key distribution device S receives (R_(i), c_(i)) orc_(i) from the communication devices U_(i). At this time, the keydistribution device S stands by until the key distribution device Sreceives (R₁, c₁), c₂, . . . , c_(n−1), (R_(n), c_(n)), . . . ,(R_(n+k), c_(n+k)) respectively from all of the communication devicesU₁, . . . , U_(n+k).

In step S313, the session ID generation unit 113 of the key distributiondevice S generates sid=H(c₁, . . . , c_(n+k)) by using c₁, . . . ,c_(n+k), which are received from the communication devices U₁, . . . ,U_(n+k), based on the target-collision resistant hash function H.Further, the session ID generation unit 113 selects one piece ofcommunication device as a representative from k+2 pieces ofcommunication devices U₁, U_(n), U_(n+1), . . . , U_(n+k). It is assumedthat the communication device U₁ is selected, and U₁ is called arepresentative communication device in this example. Further, n+k−1pieces of communication devices U_(i) (i∈{2, . . . , n+k}) other thanthe representative communication device U₁ are called generalcommunication devices. In the case of i∈[n+1, n+k], the session IDgeneration unit 113 transmits (sid, R_(i−1), R_(i+1)) to each of thecommunication devices U_(i) (here, R_(n+k+1)=R₁). Further, in the caseof i∈[1, 2], the session ID generation unit 113 transmits (sid, R_(i−1))to each of the communication devices U_(i) (here, R₀=R_(n+k)). In thecase of i∈[3, n−2], the session ID generation unit 113 transmits sid toeach of the communication devices U_(i). In the case of i∈[n−1, n], thesession ID generation unit 113 transmits (sid, R_(i+1)) to each of thecommunication devices U_(i). Further, the key distribution device Snotifies U₁ that U₁ is the representative communication device.

In step S413, each of the communication devices U_(i) receives any of(sid, R_(i−1), R_(i+1)), (sid, R_(i−1)), sid, and (sid, R_(i+1)) fromthe key distribution device S. Specifically, in the case of i∈[n+1,n+k], the communication devices U_(i) receive (sid, R_(i−1), R_(i+1))(here, R_(n+k+1)=R₁). In the case of i∈[1, 2], the communication devicesU_(i) receive (sid, R_(i−1)) (here, R₀=R_(n+k)). In the case of i∈[3,n−2], the communication devices U_(i) receive sid. In the case ofi∈[n−1, n], the communication devices U_(i) receive (sid, R_(i+1)).

The communication devices U_(i) execute the processing of step S414 andstep S415 as soon as the communication devices U_(i) receive any of(sid, R_(i−1), R_(i+1)), (sid, R_(i−1)), sid, and (sid, R_(i+1)). Thisprocessing is executed for six cases which are the case of i=1, the caseof i=2, the case of i∈[3, n−2], the case of i=n−1, the case of i=n, andthe case of i∈[n+1, n+k]. However, in the case of i∈[3, n−2], noprocessing is performed in step S414. That is, as soon as thecommunication devices U_(i) receive sid, the communication devices U_(i)execute the processing of step S415.

In the case of i=1, in step S414, the second key generation unit 214 ofthe representative communication device U₁ generates K₁ ^(l) by using(sid, R_(n+k) ^(r_1)) based on the pseudo-random function F andgenerates K₁ ^(r) by using (sid, g^(r_1r)) based on the pseudo-randomfunction F so as to compute T₁ by an exclusive OR of K₁ ^(l) and K₁ ^(r)and compute T′ by the exclusive OR of K₁ ^(l) and k₁∥s₁, as thefollowing formulas. Here, ∥ denotes a concatenation operator.K ₁ ^(l) =F(sid,R _(n+k) ^(r) ¹ ),K ₁ ^(r) =F(sid,g ^(r) ¹ ^(r)),T ₁ =K ₁ ^(l) ⊕K ₁ ^(r),T′=K ₁ ^(l)⊕(k ₁ ∥s ₁)

In step S415, the authentication tag generation unit 215 of therepresentative communication device U₁ generates an authentication tagσ₁=Tag_(mk_1)(R₁, c_(i), R_(n+k), T₁, T′, U₁, sid) by using the MAC keymk₁ based on the tag generation algorithm Tag for a messageauthentication code. The authentication tag generation unit 215transmits (T₁, T′, σ₁) to the key distribution device S.

In the case of i=2, in step S414, the second key generation unit 214 ofthe communication device U₂ generates K₂ ^(l) by using (sid, R₁ ^(r))based on the pseudo-random function F and generates K₂ ^(r) by using(sid, g^(r)) based on the pseudo-random function F so as to compute T₂by an exclusive OR of K₂ ^(l) and K₂ ^(r) as the following formulas.K ₂ ^(l) =F(sid,R ₁ ^(r)),K ₂ ^(r) =F(sid,g ^(r)),T ₂ =K ₂ ^(l) ⊕K ₂ ^(r)

In step S415, the authentication tag generation unit 215 of thecommunication device U₂ generates an authentication tagσ₂=Tag_(mk_2)(c₂, R₁, k₂, s₂, T₂, U₂, sid) by using the MAC key mk₂based on the tag generation algorithm Tag for a message authenticationcode. The authentication tag generation unit 215 transmits (k₂, s₂, T₂,σ₂) to the key distribution device S.

In the case of i∈[3, n−2], in step S415, the authentication taggeneration unit 215 of the communication devices U_(i) generates anauthentication tag σ_(i)=Tag_(mk_i)(c_(i), k_(i), s_(i), U_(i), sid) byusing the MAC key mk_(i) based on the tag generation algorithm Tag for amessage authentication code. The authentication tag generation unit 215transmits (k_(i), s_(i), σ_(i)) to the key distribution device S.

In the case of i=n−1, in step S414, the second key generation unit 214of the communication device U_(n−1) generates K_(n−1) ^(l) by using(sid, g^(r)) based on the pseudo-random function F and generates K_(n−1)^(r) by using (sid, R_(n) ^(r)) based on the pseudo-random function F soas to compute T_(n−1) by an exclusive OR of K_(n−1) ^(l) and K_(n−1)^(r), as the following formulas.K _(n−1) ^(l) =F(sid,g ^(r)),K _(n−1) ^(r) =F(sid,R _(n) ^(r)),T _(n−1) =K _(n−1) ^(l) ⊕K _(n−1) ^(r)

In step S415, the authentication tag generation unit 215 of thecommunication device U_(n−1) generates an authentication tagσ_(n−1)=Tag_(mk_n−1)(c_(n−1), R_(n), k_(n−1), s_(n−1), T_(n−1), U_(n−1),sid) by using the MAC key mk_(n−1) based on the tag generation algorithmTag for a message authentication code. The authentication tag generationunit 215 transmits (k_(n−1), s_(n−1), T_(n−1), σ_(n−1)) to the keydistribution device S.

In the case of i=n, in step S414, the second key generation unit 214 ofthe communication device U_(n) generates K_(n) ^(l) by using (sid, R_(n)^(r)) based on the pseudo-random function F and generates K_(n) ^(r) byusing (sid, R_(n+1) ^(r_n)) based on the pseudo-random function F so asto compute T_(n) by an exclusive OR of K_(n) ^(l) and K_(n) ^(r), as thefollowing formulas.K _(n) ^(l) =F(sid,R _(n) ^(r)),K _(n) ^(r) =F(sid,R _(n+1) ^(r) ^(n) ),T _(n) =K _(n) ^(l) ⊕K _(n) ^(r)

In step S415, the authentication tag generation unit 215 of thecommunication device U_(n) generates an authentication tagσ_(n)=Tag_(mk_n)(R_(n), c_(n), R_(n+1), k_(n), s_(n), T_(n), U_(n), sid)by using the MAC key mk_(n) based on the tag generation algorithm Tagfor a message authentication code. The authentication tag generationunit 215 transmits (k_(n), s_(n), T_(n), σ_(n)) to the key distributiondevice S.

In the case of i∈[n+1, n+k], in step S414, the second key generationunit 214 of the communication device U_(i) generates K_(i) ^(l) by using(sid, R_(i−1) ^(r_i)) based on the pseudo-random function F andgenerates K_(i) ^(r) by using (sid, R_(i+1) ^(r_i)) based on thepseudo-random function F so as to compute T_(i) by an exclusive OR ofK_(i) ^(l) and K_(i) ^(r), as the following formulas.K _(i) ^(l) =F(sid,R _(i−1) ^(r) ^(i) ),K _(i) ^(r) =F(sid,R _(i+1) ^(r) ^(i) ),T _(i) =K _(i) ^(l) ⊕K _(i) ^(r)

In step S415, the authentication tag generation unit 215 of thecommunication devices U_(i) generates an authentication tagσ_(i)=Tag_(mk_i)(R_(i), c_(i), R_(i−1), R_(i+1), k_(i), s_(i), T_(i),U_(i), sid) by using the MAC key mk_(i) based on the tag generationalgorithm Tag for a message authentication code. The authentication taggeneration unit 215 transmits (k_(i), s_(i), T_(i), σ_(i)) to the keydistribution device S.

In step S314, the authentication tag verification unit 114 of the keydistribution device S receives (T₁, T′, σ₁) from the representativecommunication devices U₁, receives (k_(i), s_(i), T_(i), σ_(i)) from thegeneral communication devices U_(i) with respect to i=2, n−1, . . . ,n+k, and receives (k_(i), s_(i), σ_(i)) from the general communicationdevice U_(i) with respect to i=3, . . . , n−2 so as to performverification based on the verification algorithm Ver for a messageauthentication code. When the authentication tag σ_(i) (i=1, . . . ,n+k) is invalid, the authentication tag verification unit 114 ends thesession of the communication devices U_(i). Further, the authenticationtag verification unit 114 verifies whether or not c_(i)=g^(k_i)h^(s_i)is satisfied with respect to i=2, . . . , n+k. When c_(i)=g^(k_i)h^(s_i)is not satisfied, the authentication tag verification unit 114 ends thesession of the general communication devices U_(i).

In step S315 a, the third key generation unit 115 of the keydistribution device S generates ^(˜)k_(S)∈_(R){0, 1}^(κ) and^(˜)k_(S)∈_(R)FS_(κ) so as to compute k_(S)=tPRF(^(˜)k_(S), ^(˜)k′_(S),st_(S), st′_(S)) based on the twisted pseudo-random function tPRF.Further, the third key generation unit 115 computes k′ by the followingformula.k′=(⊕_(2≤i≤n) k _(i))⊕k _(S)

In step S315 b, the third key generation unit 115 of the keydistribution device S computes T′_(i) with respect to i=2, . . . , n+kby the following formula.T′ _(i)⊕_(1≤j≤i−1) T _(j)

Here, T_(i) is nil with respect to i=3, . . . , n−1. Accordingly, T₃′= .. . =T_(n−1)′=0 is obtained.

In step S315 c, the third key generation unit 115 of the keydistribution device S encrypts a common key K₁ with respect to i=1, . .. , n+k based on the encryption algorithm FEnc for functional encryptionwith the access structure P_(i)=(ID=U_(i)){circumflex over ( )}(time∈TF)so as to generate a cipher text CT′_(i)=FEnc(Params, P_(i), K₁). Here,ID is a predicate variable representing a communication device and TF isa predicate variable representing a time frame of the communicationdevice. Further, the common key K₁ is the one generated in step S115 c.

In step S316, the key distribution device S generates an authenticationtag and transmits the authentication tag to the communication devicesU_(i). This processing is executed for six cases which are the case ofi=1, the case of i=2, the case of i∈[3, n−2], the case of i=n−1, thecase of i=n, and the case of i∈[n+1, n+k].

In the case of i=1, the authentication tag generation unit 116 of thekey distribution device S generates an authentication tagσ′₁=Tag_(mk_1)(R₁, c₁, R_(n+k), T₁, T′, U₁, sid, k′, CT′₁) by using theMAC key mk_(i) of the representative communication device U_(i) based onthe tag generation algorithm Tag for a message authentication code. Theauthentication tag generation unit 116 transmits (k′, CT′₁, σ′₁) to therepresentative communication device U₁.

In the case of i=2, the authentication tag generation unit 116 of thekey distribution device S generates an authentication tagσ′₂=Tag_(mk_2)(c₂, R₁, k₂, s₂, T₂, U₂, sid, c_(i), k′, T′₂, T′, CT′₂) byusing the MAC key mk₂ of the general communication device U₂ based onthe tag generation algorithm Tag for a message authentication code. Theauthentication tag generation unit 116 transmits (c₁, k′, T′₂, T′, CT′₂,σ′₂) to the general communication device U₂.

In the case of i∈[3, n−2], the authentication tag generation unit 116 ofthe key distribution device S generates an authentication tagσ′_(i)=Tag_(mk_i)(c_(i), k_(i), s_(i), U_(i), sid, c_(i), k′, T′_(i),T′, CT′_(i)) by using the MAC key mk_(i) of the general communicationdevices U_(i) based on the tag generation algorithm Tag for a messageauthentication code. The authentication tag generation unit 116transmits (c_(i), k′, T′_(j), T′, CT′_(j), σ′_(i)) to the generalcommunication device U_(i).

In the case of i=n−1, the authentication tag generation unit 116 of thekey distribution device S generates an authentication tagσ′_(n−1)=Tag_(mk_n−1)(c_(n−1), R_(n), k_(n−1), s_(n−1), T_(n−1),U_(n−1), sid, c₁, k′, T′_(n−1), T′, CT′_(n)) by using the MAC keymk_(n−1) of the general communication device U_(n−1) based on the taggeneration algorithm Tag for a message authentication code. Theauthentication tag generation unit 116 transmits (c₁, k′, T′_(n−1), T′,CT′_(n−1), σ′_(n−1)) to the general communication device U_(n−1).

In the case of i=n, the authentication tag generation unit 116 of thekey distribution device S generates an authentication tagσ′_(n)=Tag_(mk_n)(R_(n), c_(n), R_(n+1), k_(n), s_(n), T_(n), U_(n),sid, c₁, k′, T′_(n), T′, CT′_(n)) by using the MAC key ink, of thegeneral communication devices U_(n) based on the tag generationalgorithm Tag for a message authentication code. The authentication taggeneration unit 116 transmits (c₁, k′, T′_(n), T′, CT′_(n), σ′_(n)) tothe general communication device U_(n).

In the case of i∈[n+1, n+k], the authentication tag generation unit 116of the key distribution device S generates an authentication tagσ′_(i)=Tag_(mk_i)(R_(i), c_(i), R_(i−1), R_(i+1), k_(i), s_(i), T_(i),U_(j), sid, c_(i), k′, T′_(i), T′, CT′_(i)) by using the MAC key mk_(i)of the general communication devices U_(i) based on the tag generationalgorithm Tag for a message authentication code. The authentication taggeneration unit 116 transmits (c₁, k′, T′_(i), T′, CT′_(i), σ′_(i)) tothe general communication devices U_(i).

In step S416, in the case of i∈[2, n+k], the authentication tagverification unit 216 of the general communication devices U_(i)receives (c₁, k′, T′_(i), T′, CT′_(i), σ′_(i)) from the key distributiondevice S and performs verification based on the verification algorithmVer for a message authentication code. When the authentication tagσ′_(i) is invalid, the authentication tag verification unit 216 ends thesession of the general communication devices U_(i). Further, theauthentication tag verification unit 216 computes K₁ ^(l) by anexclusive OR of T′_(i) and K_(i) ^(l) with respect to i=2, n, . . . ,n+k and computes K₁ ^(l) by an exclusive OR of T′_(i) and g^(r) withrespect to i=3, . . . , n−1.K ₁ ^(l) =T′ _(i) ⊕K _(i) ^(l)K ₁ ^(l) =T′ _(i) ⊕g ^(r)

Further, the authentication tag verification unit 216 computes k₁∥s₁ byan exclusive OR of T′ and K₁ ^(l).k ₁ ∥s ₁ =T′⊕K ₁ ^(l)

Then, the authentication tag verification unit 216 verifies whether ornot c₁=g^(k_1)h^(s_1) is satisfied. When c₁=g^(k_1)h^(s_1) is notsatisfied, the authentication tag verification unit 216 ends the sessionof the general communication devices U_(i).

In the case of i=1, the authentication tag verification unit 216 of therepresentative communication device U₁ receives (k′, CT′₁, σ′₁) from thekey distribution device S and performs verification based on theverification algorithm Ver for a message authentication code. When theauthentication tag σ′₁ is invalid, the authentication tag verificationunit 216 ends the session of the representative communication device U₁.

In step S417, the session key generation unit 217 of the communicationdevices U_(i) decrypts the common key K₁←FDec_(usk_i)(CT′_(i), P_(i)) byusing the user secret key usk_(i) of the communication devices U_(i)based on the decryption algorithm FDec for functional encryption.Further, the session key generation unit 217 computes a common key K₂based on the pseudo-random function F′ as the following formula.K ₂ =F′(sid,k′⊕k ₁)

Then, the session key generation unit 217 computes a session key SKbased on the pseudo-random function F″ as the following formula.SK=F″(sid,K ₁)⊕F″(sid,K ₂)

Last, the communication devices U_(i) (i=1, . . . , n) update secretinformation to be used for user addition. Further, the communicationdevices U_(i)(i=n+1, . . . , n+k) newly store the secret information inthe storage 200. The representative communication devices U₁ update thesecret information stored in the storage 200 with secret information H₁^(l) and r computed by the following formulas.H ₁ ^(l) =R _(n+k) ^(r) ¹r=F′″(sid,K ₁)⊕F′″(sid,K ₂)

The communication device U_(i) (i=2, . . . , n−1) updates the secretinformation stored in the storage 200 with secret information r computedby the following formula.r=F′″(sid,K ₁)⊕F′″(sid,K ₂)

The communication devices U_(n) update the secret information stored inthe storage 200 with secret information H_(n) ^(r) and r computed by thefollowing formulas.H _(n) ^(r) =R _(n+1) ^(r) ^(n)r=F′″(sid,K ₁)⊕F′″(sid,K ₂)

The communication device U_(i) (i=n+1, . . . , n+k) stores secretinformation H_(i) ^(l), H_(i) ^(r), and r computed by the followingformulas in the storage 200.H _(i) ^(l) =R _(i−1) ^(r) ¹H _(i) ^(r) =R _(i+1) ^(r) ^(i)r=F′″(sid,K ₁)⊕F′″(sid,K ₂)

According to the key exchange technology of the present invention, ifonly user IDs are preliminarily registered as information of users whomay perform key exchange, users can be dynamically added within thecorresponding range, in the above-described configuration.

Further, a plurality of users can efficiently share the common key alsoin the user addition. Specifically, the whole computational complexityrequired for the key exchange is the constant number of times which isthe number of users, that is, O(1).

Application Example

The key exchange system according to the present embodiment is thesystem in which the session key SK is shared among the communicationdevices U_(i) via the key distribution device S. In this case, thoughuser IDs for identifying the communication devices U_(i) (for example,mail addresses) need to be preliminarily registered in the keydistribution device S, the session key SK can be shared in O(1) by usinga star topology and therefore, time until the start of operations ofvarious encryption communication systems can be shortened. Someapplication examples of this key exchange system will be shown below.

(1) To use for key distribution in a call between two points and atelephone conference employing the VoIP technology.

(2) To use for key distribution in encryption communication in a Webconference and a Web call using browser and smartphones, in place ofdatagram transport layer security (DTLS) in WebRTC.

(3) To use for sharing a decryption key for a messaging application.

Modification

It is obvious that the present invention is not limited to theabove-described embodiment and alterations can be made as appropriatewithin a scope of the idea of the present invention. Various types ofprocessing which are described in the above embodiment may be executedin time series in accordance with the described order and may beexecuted in parallel or individually in accordance with the processingcapacity of the device performing the processing or in accordance withthe need.

[Program and Recording Medium]

When various types of processing functions in the devices described inthe above embodiment are implemented on a computer, the contents ofprocessing function to be contained in each device is written by aprogram.

With this program executed on the computer, various types of processingfunctions in the above-described devices are implemented on thecomputer.

This program in which the contents of processing are written can berecorded in a computer-readable recording medium. The computer-readablerecording medium may be any medium such as a magnetic recording device,an optical disc, a magneto-optical recording medium, and a semiconductormemory.

Distribution of this program is implemented by sales, transfer, rental,and other transactions of a portable recording medium such as a DVD anda CD-ROM on which the program is recorded, for example.

Furthermore, this program may be stored in a storage unit of a servercomputer and transferred from the server computer to other computers viaa network so as to be distributed.

A computer which executes such program first stores the program recordedin a portable recording medium or transferred from a server computeronce in a storage unit of the computer, for example. When the processingis performed, the computer reads out the program stored in the storageunit of the computer and performs processing in accordance with theprogram thus read out. As another execution form of this program, thecomputer may directly read out the program from a portable recordingmedium and perform processing in accordance with the program.

Furthermore, each time the program is transferred to the computer fromthe server computer, the computer may sequentially perform processing inaccordance with the received program. Alternatively, a configuration maybe adopted in which the transfer of a program to the computer from theserver computer is not performed and the above-described processing isexecuted by so-called application service provider (ASP)-type service bywhich the processing functions are implemented only by an instructionfor execution thereof and result acquisition. It should be noted that aprogram according to the present embodiment includes information whichis provided for processing performed by electronic calculation equipmentand which is equivalent to a program (such as data which is not a directinstruction to the computer but has a property specifying the processingperformed by the computer).

In the present embodiment, the present device is configured with apredetermined program executed on a computer. However, the presentdevice may be configured with at least part of these processing contentsrealized in a hardware manner.

The foregoing description of the embodiment of the invention has beenpresented for the purpose of illustration and description. It is notintended to be exhaustive and to limit the invention to the precise formdisclosed. Modifications or variations are possible in light of theabove teaching. The embodiment was chosen and described to provide thebest illustration of the principles of the invention and its practicalapplication, and to enable one of ordinary skill in the art to utilizethe invention in various embodiments and with various modifications asare suited to the particular use contemplated. All such modificationsand variations are within the scope of the invention as determined bythe appended claims when interpreted in accordance with the breadth towhich they are fairly, legally, and equitably entitled.

DESCRIPTION OF REFERENCE NUMERALS

-   -   1 key distribution device    -   2 communication device    -   3 communication network    -   100 storage    -   101 first setup unit    -   102 second setup unit    -   103 user ID reception unit    -   104 initial key generation unit    -   105 secret string generation unit    -   111 user key transmission unit    -   113 session ID generation unit    -   114 authentication tag verification unit    -   115 third key generation unit    -   116 authentication tag generation unit    -   200 storage    -   203 user ID transmission unit    -   204 initial key reception unit    -   205 secret string generation unit    -   211 user key reception unit    -   212 first key generation unit    -   214 second key generation unit    -   215 authentication tag generation unit    -   216 authentication tag verification unit    -   217 session key generation unit

What is claimed is:
 1. A key exchange method for a case wherecommunication devices U_(n+1), . . . , U_(n+k) newly join a sessionestablished by communication devices U₁, . . . , U_(n), when n denotesan integer which is 2 or larger, k denotes an integer which is 1 orlarger, ∥ denotes a concatenation operator, S denotes a key distributiondevice, and U_(i) (i=1, . . . , n+k) denotes n+k pieces of communicationdevices, in which U₁ denotes one piece of representative communicationdevice which is selected from the communication devices U₁, U_(n),U_(n+1), . . . , U_(n+k), secret strings st_(S) and st′_(S) are storedin a storage of the key distribution device S, secret strings st_(i) andst′_(i) are stored in a storage of the communication devices U_(i) (i=1,. . . , n+k), and secret information r, which is generated in thesession established by is communication devices U₁, . . . , U_(n), isstored in the storage of the communication devices U₁, . . . , U_(n),the key exchange method comprising: a first key generation step in whichthe communication devices U_(i) (i=1, n, . . . , n+k) generate r_(i),k_(i), and s_(i) by using the secret strings st_(i) and st′_(i) based ona twisted pseudo-random function, compute R_(i)=g^(r_i) andc_(i)=g^(k_i)n^(s_i), and transmit (R_(i), c_(i)) to the keydistribution device S, and the communication devices U_(i)(i=2, . . . ,n−1) generate k_(i) and s_(i) by using the secret strings st_(i) andst′_(i) based on the twisted pseudo-random function, computec_(i)=g^(k_i)h^(s_i), and transmit c_(i) to the key distribution deviceS; a session ID generation step in which the key distribution device Sgenerates sid by using c₁, . . . , c_(n+k) based on a target-collisionresistant hash function and transmits, to the communication devicesU_(i), (sid, R_(i−1)) with respect to i=1, 2, sid with respect to i=3, .. . , n−2, (sid, R_(i+1)) with respect to i=n−1, n, and (sid, R_(i−1),R_(i+1)) with respect to i=n+1, . . . , n+k (here, R₀=R_(n+K) andR_(n+k+1)=R₁); a second key generation step in which the representativecommunication device U₁ generates K₁ ^(l) by using (sid, R_(n+k) ^(r_1))based on a pseudo-random function, generates K₁ ^(l) by using (sid,g^(r_1r)) based on the pseudo-random function, computes T₁ by anexclusive OR of K₁ ^(l) and K₁ ^(r), computes T′ by an exclusive OR ofK₁ ^(l) and k₁∥s₁, and transmits (T₁, T′) to the key distribution deviceS, the communication device U₂ generates K₂ ^(l) by using (sid, R₁ ^(r))based on the pseudo-random function, generates K₂ ^(r) by using (sid,g^(r)) based on the pseudo-random function, computes T₂ by an exclusiveOR of K₂ ^(l) and K₂ ^(r), and transmits (k₂, s₂, T₂) to the keydistribution device S, the communication devices U_(i) (i=3, . . . ,n−2) transmit (k_(i), s_(i)) to the key distribution device S, thecommunication device U_(n−1) generates K_(n−1) ^(l) by using (sid,g^(r)) based on the pseudo-random function, generates K_(n−1) ^(r) byusing (sid, R_(n) ^(r)) based on the pseudo-random function, computesT_(n−1) by an exclusive OR of K_(n−1) ^(l) and K_(n−1) ^(r), andtransmits (k_(n−1), s_(n−1), T_(n−1)) to the key distribution device S,the communication device U_(n) generates K_(n) ^(l) by using (sid, R_(n)^(r)) based on the pseudo-random function, generates K_(n) ^(r) by using(sid, R_(n+1) ^(r_n)) based on the pseudo-random function, computesT_(n) by an exclusive OR of K_(n) ^(l) and K_(n) ^(r), and transmits(k_(n), s_(n), T_(n)) to the key distribution device S, and thecommunication devices U_(i)(i=n+1l, . . . , n+k) generate K_(i) ^(l) byusing (sid, R_(i−1) ^(r_1)) based on the pseudo-random function,generate K_(i) ^(r) by using (sid, R_(i+1) ^(r_i)) based on thepseudo-random function, compute T_(i) by an exclusive OR of K_(i) ^(l)and K_(i) ^(r), and transmit (k_(i), s_(i), T_(i)) to the keydistribution device S; a third key generation step in which the keydistribution device S generates k_(s) by using the secret strings st_(S)and st′_(S) based on the twisted pseudo-random function, computes k′ byan exclusive OR of k₂, . . . , k_(n+k), k_(s), computes T′_(i) by anexclusive OR of T₁, . . . , T_(i−1) with respect to i=2, . . . , n+k(here, T_(i) is nil with respect to i=3, . . . , n−1), transmits k′ tothe representative communication device U_(i), and transmits (k′,T′_(i), T′) to the communication devices U_(i) (i=2, . . . , n+k); afirst session key generation step in which the communication devicesU_(i)(i=2, n, . . . , n+k) compute K₁ ^(l) by an exclusive OR of T′_(i)and K_(i) ^(l) and compute k₁∥s₁ by an exclusive OR of T′ and K₁ ^(l),and the communication devices U_(i) (i=3, . . . , n−1) compute K₁ ^(l)by an exclusive OR of T′_(i) and g^(r) and compute k₁∥s₁, by theexclusive OR of T′ and K₁ ^(l); and a second session key generation stepin which the communication devices U_(i) (i=1, . . . , n+k) generate acommon key K₂ by using sid and an exclusive OR of k′ and k₁ based on thepseudo-random function.
 2. The key exchange method according to claim 1,in which time denotes current time, ID denotes a predicate variablerepresenting the communication device, and TF denotes a predicatevariable representing a time frame of the communication device, a mastersecret key msk for functional encryption and a common key K₁ which isgenerated in a session established by the communication devices U₁, . .. , U_(n) are further stored in the storage of the key distributiondevice S, and an initial key sk_(U_i) which is generated from anidentifier U_(i) of the communication devices U_(i) by using the mastersecret key msk′ based on a key derivation algorithm for ID-baseencryption is further stored in the storage of the communication devicesU_(i) (i=1, . . . , n+k), the key exchange method further comprising: auser key transmission step in which the key distribution device Sgenerates a user secret key usk_(i) by using the master secret key mskwith respect to i=1, . . . , n+k with attribution used as A_(i)=(U_(i),time) based on a key derivation algorithm for functional encryption, andencrypts the user secret key usk_(i) by using the identifier U_(i) basedon an encryption algorithm for ID-base encryption so as to generate acipher text CT_(i); and a user key reception step in which thecommunication devices U_(i) (i=1, . . . , n+k) decrypt the cipher textCT_(i) by using the initial key sk_(U_i) based on a decryption algorithmfor ID-base encryption so as to obtain the user secret key usk_(i),wherein in the third key generation step, the common key K₁ stored inthe storage of the key distribution device S is read and the common keyK₁ is encrypted with respect to i=1, . . . , n+k with an accessstructure P_(i)=(ID=U_(i)){circumflex over ( )}(time∈TF) based on anencryption algorithm for functional encryption so as to generate acipher text CT′_(i), and in the second session key generation step, thecipher text CT′_(i) is decrypted by using the user secret key usk_(i)based on a decryption algorithm for functional encryption so as toobtain the common key K₁ and a session key SK is computed by anexclusive OR of a value generated by using (sid, K₁) based on thepseudo-random function and a value generated by using (sid, K₂) based onthe pseudo-random function.
 3. The key exchange method according toclaim 2, in which in the user key transmission step, a MAC key mk_(i) isgenerated based on a key generation algorithm for a messageauthentication code and the user secret key usk_(i) and the MAC keymk_(i) are encrypted by using the identifier U_(i) based on theencryption algorithm for ID-base encryption so as to generate the ciphertext CT_(i), and in the user key reception step, the cipher text CT_(i)is decrypted by using the initial key sk_(U_i) based on the decryptionalgorithm for ID-base encryption so as to obtain the user secret keyusk_(i) and the MAC key mk_(i), the key exchange method furthercomprising: a first authentication tag generation step in which therepresentative communication device U₁ generates an authentication tagσ₁ by using the MAC key mk₁ and R₁, c_(i), R_(n+k), T₁, T′, U₁, and sidbased on a tag generation algorithm for a message authentication code,the communication device U₂ generates an authentication tag σ₂ by usingthe MAC key mk₂ and c₂, R₁, k₂, s₂, T₂, U₂, and sid based on the taggeneration algorithm for a message authentication code, thecommunication devices U_(i) (i=3, . . . , n−2) generate anauthentication tag σ_(i) by using the MAC key mk_(i) and c_(i), k_(i),s_(i), U_(i), and sid based on the tag generation algorithm for amessage authentication code, the communication device U_(n−1) generatesan authentication tag σ_(n−1) by using the MAC key mk_(n−1) and c_(n−1),R_(n), k_(n−1), s_(n−1), T_(n−1), U_(n−1), and sid based on the taggeneration algorithm for a message authentication code, thecommunication device U_(n) generates an authentication tag σ_(n) byusing the MAC key mk_(n) and R_(n), c_(n), R_(n+1), k_(n), s_(n), T_(n),U_(n), and sid based on the tag generation algorithm for a messageauthentication code, and the communication devices U_(i) (i=n+1, . . . ,n+k) generate an authentication tag σ_(i) by using the MAC key mk_(i)and R_(i), c_(i), R_(i−1), R_(i+1), k_(i), s_(i), T_(i), U_(i), and sidbased on the tag generation algorithm for a message authentication code;a first authentication tag verification step in which the keydistribution device S receives (T₁, T′, σ₁) from the representativecommunication device U₁, receives (k_(i), s_(i), T_(i), σ_(i)) from thecommunication devices U_(i) (i=2, n−1, . . . , n+k), receives (k_(i),s_(i), σ_(i)) from the communication devices U_(i) (i=3, . . . , n−2),verifies the authentication tag σ_(i) by using the MAC key mk_(i) (i=1,. . . , n+k) based on a verification algorithm for a messageauthentication code, and verifies whether or not c_(i)=g^(k_i)h^(s_i) issatisfied with respect to i=2, . . . , n+k; a second authentication taggeneration step in which the key distribution device S generates anauthentication tag σ′₁ with respect to i=1 by using the MAC key mk₁ andR₁, c₁, R_(n+k), T₁, T′, U₁, sid, k′, and CT′₁ based on the taggeneration algorithm for a message authentication code, generates anauthentication tag σ′₂ with respect to i=2 by using the MAC key mk₂ andc₂, R₁, k₂, s₂, T₂, U₂, sid, c₁, k′, T′₂, T′, and CT′₂ based on the taggeneration algorithm for a message authentication code, generates anauthentication tag σ′_(i) with respect to i=3, . . . , n−2 by using theMAC key mk_(i) and c_(i), k_(i), s_(i), U_(i), sid, c₁, k′, T′_(i), T′,and CT'S based on the tag generation algorithm for a messageauthentication code, generates an authentication tag σ′_(n−1) withrespect to i=n−1 by using the MAC key mk_(n−1) and c_(n−1), R_(n),k_(n−1), s_(n−1), T_(n−1), U_(n−1), sid, c₁, k′, T′_(n−1), T′, andCT′_(n−1) based on the tag generation algorithm for a messageauthentication code, generates an authentication tag σ′_(n) with respectto i=n by using the MAC key mk_(n) and R_(n), c_(n), R_(n+1), k_(n),s_(n), T_(n), U_(n), sid, c₁, k′, T′_(n), T′, and CT′_(n) based on thetag generation algorithm for a message authentication code, andgenerates an authentication tag σ′_(i) with respect to i=n+1, . . . ,n+k by using the MAC key mk_(i) and R_(i), c_(i), R_(i−1), R_(i+1),k_(i), s_(i), T_(i), U_(i), sid, c₁, k′, T′_(i), T′, and CT′_(i) basedon the tag generation algorithm for a message authentication code; and asecond authentication tag verification step in which the representativecommunication device U₁ receives (k′, CT′₁, σ′₁) from the keydistribution device S and verifies the authentication tag σ′₁ by usingthe MAC key mk₁ based on the verification algorithm for a messageauthentication code, and the communication devices U_(i) (i=2, . . . ,n+k) receive (c_(i), k′, T′_(i), T′, CT′_(i), σ′_(i)) from the keydistribution device S, verify the authentication tag σ′_(i) by using theMAC key mk_(i) based on the verification algorithm for a messageauthentication code, and verify whether or not c₁=g^(k_1)h^(s_1) issatisfied.
 4. A key exchange system in which n denotes an integer whichis 2 or larger, k denotes an integer which is 1 or larger, and ∥ denotesa concatenation operator, the key exchange system comprising: a keydistribution device S; and n+k pieces of communication devices U_(i)(i=1, . . . , n+k), wherein U₁ denotes one piece of representativecommunication device which is selected from the communication devicesU₁, U_(n), U_(n+1), . . . , U_(n+k), the key distribution device Sincludes a storage which stores secret strings st_(S) and st′_(S), asession ID generation unit which receives (R_(i), c_(i)) from thecommunication devices U_(i) (i=1, n, . . . , n+k), receives c_(i) fromthe communication devices U_(i) (i=2, . . . , n−1), generates sid byusing c₁, . . . , c_(n+k) based on a target-collision resistant hashfunction, and transmits, to the communication devices U_(i), (sid,R_(i−1)) with respect to i=1, 2, sid with respect to i=3, . . . , n−2,(sid, R_(i+1)) with respect to i=n−1, n, and (sid, R_(i−1), R_(i+1))with respect to i=n+1, . . . , n+k (here, R₀=R_(n+K) and R_(n+k+1)=R₁),and a third key generation unit which receives (T₁, T′) from therepresentative communication device U₁, receives (k_(i), s_(i), T_(i))from the communication device U_(i) (i=2, n−1, . . . , n+k), receives(k_(i), s_(i)) from the communication devices U_(i) (i=3, . . . , n−2),generates k_(s) by using the secret strings st_(S) and st′_(S) based ona twisted pseudo-random function, computes k′ by an exclusive OR of k₂,. . . , k_(n+k), k_(s), computes T′_(i) by an exclusive OR of T₁, . . ., T_(i−1) with respect to i=2, . . . , n+k (here, T_(i) is nil withrespect to i=3, . . . n−1), transmits k′ to the representativecommunication device U₁, and transmits (k′, T′_(i), T′) to thecommunication devices U_(i) (i=2, . . . , n+k), and the communicationdevices U_(i) (i=1, . . . , n+k) include a storage which stores secretstrings st_(i) and st′_(i) and further stores secret information rgenerated in a session established by the communication devices U₁, . .. , U_(n) with respect to i=1, . . . , n, a first key generation unitwhich generates r_(i), k_(i), and s_(i) by using the secret stringsst_(i) and st′_(i) based on the twisted pseudo-random function, computesR_(i)=g^(r_i) and c_(i)=g^(k_i)h^(s_i), and transmits (R_(i), c_(i)) tothe key distribution device S with respect to i=1, n, . . . , n+k, andgenerates k_(i) and s_(i) by using the secret strings st_(i) and st′_(i)based on the twisted pseudo-random function, computesc_(i)=g^(k_i)h^(s_i), and transmits c_(i) to the key distribution deviceS with respect to i=2, . . . , n−1, a second key generation unit whichreceives (sid, R_(n+k)) from the key distribution device S, generates K₁^(l) by using (sid, R_(n+k) ^(r_1)) based on the pseudo-random function,generates K₁ ^(r) by using (sid, g^(r_1r)) based on the pseudo-randomfunction, computes T₁ by an exclusive OR of K₁ ^(l) and K₁ ^(r),computes T′ by an exclusive OR of K₁ ^(l) and k₁∥s₁, and transmits (T₁,T′) to the key distribution device S with respect to i=1, receives (sid,R₁) from the key distribution device S, generates K₂ ^(l) by using (sid,R₁ ^(r)) based on the pseudo-random function, generates K₂ ^(r) by using(sid, g^(r)) based on the pseudo-random function, computes T₂ by anexclusive OR of K₂ ^(l) and K₂ ^(r), and transmits (k₂, s₂, T₂) to thekey distribution device S with respect to i=2, receives sid from the keydistribution device S and transmits (k_(i), s_(i)) to the keydistribution device S with respect to i=3, . . . , n−2, receives (sid,R_(n)) from the key distribution device S, generates K_(n−1) ^(l) byusing (sid, g^(r)) based on the pseudo-random function, generatesK_(n−1) ^(r) by using (sid, R_(n) ^(r)) based on the pseudo-randomfunction, computes T_(n−1) by an exclusive OR of K_(n−1) ^(l) andK_(n−1) ^(r), and transmits (k_(n−1), s_(n−1), T_(n−1)) to the keydistribution device S with respect to i=n−1, receives (sid, R_(n+1))from the key distribution device S, generates K_(n) ¹ by using (sid,R_(n) ^(r)) based on the pseudo-random function, generates K_(n) ^(r) byusing (sid, R_(n+1) ^(r_n)) based on the pseudo-random function,computes T_(n) by an exclusive OR of K_(n) ^(l) and K_(n) ^(r), andtransmits (k_(n), s_(n), T_(n)) to the key distribution device S withrespect to i=n, and receives (sid, R_(i−1), R_(i+1)) from the keydistribution device S, generates K_(i) ^(l) by using (sid, R_(i−1)^(r_i)) based on the pseudo-random function, generates K_(i) ^(r) byusing (sid, R_(i+1) ^(r_i)) based on the pseudo-random function,computes T_(i) by an exclusive OR of K_(i) ^(l) and K_(i) ^(r), andtransmits (k_(i), s_(i), T_(i)) to the key distribution device S withrespect to i=n+1, . . . , n+k, and a session key generation unit whichreceives k′ from the key distribution device S and generates a commonkey K₂ by using sid and an exclusive OR of k′ and k₁ based on thepseudo-random function with respect to i=1, receives (k′, T′_(i), T′)from the key distribution device S, computes K₁ ^(l) by an exclusive ORof T′_(i) and K_(i) ^(l), computes k₁∥s₁ by an exclusive OR of T′ and K₁^(l), and generates the common key K₂ by using sid and the exclusive ORof k′ and k₁ based on the pseudo-random function with respect to i=2, n,. . . , n+k, and receives (k′, T′_(i), T′) from the key distributiondevice S, computes K₁ ^(l) by an exclusive OR of T′_(i) and g^(r),computes k₁∥s₁ by an exclusive OR of T′ and K₁ ^(l), and generates thecommon key K₂ by using sid and the exclusive OR of k′ and k₁ based onthe pseudo-random function with respect to i=3, . . . , n−1.